The manufacture of sugar from sugar beets generally occurs in five stages: (1) diffusion, (2) juice purification, (3) evaporation, (4) crystallization, and (5) recovery of sugar from molasses.
The composition of sugar beets is dependent on the genetic strain, soil and fertilization factors, weather conditions during growth, incidence of plant disease, degree of maturity and the treatment between harvesting and processing. Typically, the percentage of sugar in mature beets ranges from about ten to about twenty-two percent with about ten to about sixteen percent being the medium values. A mature beet generally contains about three-quarters water. The beet has a liquid or juice phase and an insoluble or solid phase. The juice contains approximately 25 percent by weight of dissolved solids, the largest component of which is sucrose.
Sucrose, as well as other water soluble constituents, are diffused from sugar beets by a countercurrent process in which the sugar beets, sliced into thin strips called cossettes, enter one end of the diffuser while warm water enters the other. In this manner, about 98 percent of the sucrose in the sugar beet is removed along with a considerable portion of other water soluble components as well as colloidal and cell wall particles. This sugar laden juice is called "diffusion juice."
The nature and amount of the non-sucrose components in the diffusion juice greatly determines the amount of sugar that can be recovered by the crystallization portion of the process. Sugar not recovered by crystallization becomes a part of molasses, a by-product of lesser value. Molasses is marketed as an animal feed or fermentation process feed. Alternatively, it can be employed in the ion exclusion process to recover a portion of the sucrose.
Since the late 1800's, the classical process for purifying diffusion juice has been by a lime-carbon dioxide purification process. This process includes the steps of: (1) preliming or pre-defecation, (2) main-liming, (3) first carbonation, (4) solids separation and (5) second carbonation.
The purpose of preliming is to alkalize the juice to stabilize the colloidal and particulate material therein and to precipitate certain of the non-sugars. Such non-sugars include acid anion groups of relatively insoluble lime salts such as phosphate, sulfate and certain of the organic acids, proteins, and their moieties and colloidal substances which are not adequately removed in main-liming. In main liming, additional amounts of liming agent are supplied to the prelimed juice to increase the pH. During main liming, invert sugar (glucose and fructose) are destroyed and amides (glutamine and asparagine) are saponified.
After main liming, the first carbonation proceeds. In first carbonation, carbon dioxide gas is reacted with the main limed juice to precipitate added lime as calcium carbonate and bring the alkalinity of the juice to a desired low level of about 0.1 weight percent liming agent. Additional purification is accomplished when non-sugars precipitated in main liming are occluded within the growing calcium carbonate crystal or adsorbed on the crystal surface. It is further necessary at this time to remove precipitated calcium carbonate, called "carbonation mud", in order that the precipitated non-sugars do not dissolve and reenter the purified sugar solution during the second carbonation stage.
Carbonation mud separation is generally accomplished in two steps. In the first step, the carbonated juice enters a settler where the carbonation mud settles to the bottom. The supernatant is decanted to the second carbonation chamber. The settled carbonation mud is filtered to remove the carbonated juice from the mud particles. The filtrate is then returned to the first carbonation chamber or is used as a carrier for the liming agent.
In the second carbonation chamber, juice is again reacted with carbon dioxide gas to remove as much remaining lime as possible. Juice alkalinity is reduced to about 0.01 weight percent liming agent. A second carbonation juice is then filtered to remove calcium carbonate precipitate. The second carbonation filtrate may be treated with a sulfur dioxide source to inhibit color increase by the Maillard reactions.